Adelie penguins in east Antarctica. Although most melting of continent's ice is happening in the west, even the east is now shedding ice
Photograph: STAFF/REUTERS

The rise in sea levels seen over the past century is unmatched by any period in the past 6,000 years, according to a lengthy analysis of historical sea level trends.

The reconstruction of 35,000 years of sea level fluctuations finds that there is no evidence that levels changed by more than 20cm in a relatively steady period that lasted between 6,000 years ago and about 150 years ago.

This makes the past century extremely unusual in the historical record, with about a 20cm rise in global sea levels since the start of the 20th century. Scientists have identified rising temperatures, which have caused polar ice to melt and thermal expansion of the sea, as a primary cause of the sea level increase.

A two-decade-long collection of about 1,000 ancient sediment samples off Britain, north America, Greenland and the Seychelles formed the basis of the research, led by the Australian National University and published in PNAS.

The 35,000-year span of the study was chosen as this comprises an interglacial period. Researchers could pick submerged sediments that may include tree roots, suggesting a previously lower sea level, or mollusks, which can be measured against the fossil record to determine the previous sea level.

Ice started melting about 16,000 years ago, with this melting ending about 8,000 years ago. A slowdown in sea level changes didn’t occur until 6,000 years ago, however.

“It’s like if you leave a big block of ice on the table, it doesn’t melt instantaneously, there’s always a delay in the system,” said Kurt Lambeck, who led the research at ANU.

“We know from the last interglacial period that when temperatures were several degrees warmer than today there was a lot more water in the oceans, with levels around 4 to 5m higher than today. The question is how fast that change occurs when you increase temperatures.”

Lambeck said the sea level increase of the past 100 years is “beyond dispute”, backed up by separate data from salt flats and also changes to the sea floor caused by the extra weight of water.

“What we’ve seen is unusual, certainly unprecedented for these interglacial periods,” he said.

“All the studies show that you can’t just switch off this process. Sea levels will continue to rise for some centuries to come even if we keep carbon emissions at present day levels.

“What level that will get to, we are less sure about. But it’s clear we can’t just reverse the process overnight.”